A heavy duty US 2010/Euro VI type emission control system typically consists of diesel oxidation catalyst (DOC), catalyzed soot filter (CSF), urea based selective catalytic NOx reduction (SCR) and NH3 slip control catalyst (AMOX - excluded for this study). The advent of the US 2014 Green House Gas (GHG) rules has established a limit for tailpipe N2O emissions for on-road heavy duty Diesel engines, thus creating new challenges for catalyst design and system/engine calibration.In this paper, we discuss the effects of both catalyst system design and engine calibration on the formation of N2O across SCR catalysts. This study consisted of system testing on engine, modeling and component reactor testing. These three tools were used to evaluate how NO2 to NOx ratio and ammonia to NOx ratio (ANR) affect N2O formation.The study showed that all of the reviewed factors affect tailpipe N2O emissions. The amount of NO2 entering the SCR showed the greatest affect, particularly when the NO2 to NOx ratio is greater than 50%. Increasing the ANR also increased the N2O but to a lesser degree than the NO2/NOx ratio. The effect that the ANR had on N2O was also dependent on the temperature of the test cycle which is confirmed by reactor data showing the optimal temperature for forming N2O peaked at approximately 250°C.